Gravity (1). Mr Newton figured that out under the apple tree long time ago. Something that has a mass like an airplane is attracted to the other mass in close proximity i.e. our globe. This attraction makes these two subjects to come together. In aviation we will call that moment "a landing". This force would make every airplane to not stay in the air at all if it is not balanced by the second force, called "lift"

Lift (2). When the air moves around the airplane wings, or maybe I should say when the airplane wings move through the air, a lift is created. Not going into all complicated explanations about angle of attack, laminar flow, pressure difference, vortex, blah, blah, blah... the lift is the force pulling the wing up and perpendicular to its top surface. If lift equals the gravity force, the plane does not change altitude. If lift is stronger than gravity, the airplane climbs and if lift is weaker than gravity the plane descends. We still can not control gravity so the only way to affect airplane altitude would be by controlling the lift. But I will talk about it later.

Drag (3). On order to create the lift the airplane must move through the air. So it does. But it does not come free. An air does not like when something moves through it, and resists any way it can. That resistance is a force called a drag. It always pulls the subject (i.e. airplane) in opposite direction of subject's movement. The drag is equal zero only if the airplane is standing still in the air, like parked on the tarmac, for example.

Thrust (4). In order to keep an airplane moving through the air without slowing down we need to apply the force that is opposite equal to the drag. That force is called thrust and it is created by the propeller or jet engine. If the thrust force is equal in value and opposite in direction to the drag force, the airplane does not change it's speed. In order to slow down, you need to reduce thrust (for example by reducing engine power) or increase drag (for example by deploying flaps or spoilers). To speed up the plane you need to to opposite.

So we have all four forces equal and the airplane is flying level at constant speed. But it can not last forever. Eventually we would like to change the airplane direction, altitude or speed. We have four major controls to do that. Yes, good guess, there are four of them.